dist-001 c2splitter basic

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Dist-001 Revised: Oct 10, 2012

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Distillation of Close Boiling Components

with Aspen Plus® V8.0

1.

Lesson Objectives

RadFrac distil lation modeling

Column profiles

Custom stream results

Material balance across disti llation column

2. Prerequisites

Aspen Plus V8.0 Experience inserting blocks and connecting streams in Aspen Plus

Introduction to vapor liquid equilibrium

3. Background

Ethylene is an important monomer, and is made from ethane. The conversion of the reaction is not perfect, so

the ethylene must be separated from the system. Ethane and ethylene are molecularly similar, and so are

difficult to separate. The difficulty of the separation is compounded by the fact that polymer production

requires extremely pure feedstocks.

The examples presented are solely intended to illustrate specifi c concepts and principles. They may not

reflect an industrial application or real situation.

4. Problem Statement and Aspen Plus Solution

Problem: A stream containing 68.5wt% ethylene with a total flowrate of 7.3 million lb/day is fed into a

distillation column consisting of 125 stages. It is desired to produce a distillate product stream containing a

minimum of 99.96 wt% ethylene with a total flowrate of 5 million lb/day. It is also desired that the bottoms

product contains no more than 0.10wt% ethylene. Determine if this separation is feasible.




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Assumptions:

- 100% tray efficiency

- Total condenser

- 300 psig column operating pressure

-

A refrigerant utility stream capable of condensing the ethylene mixture (not included in model)- Feed mixture is at 350 psig and is a vapor

- 125 stages

- Feed enters column at stage 90

- Peng-Robinson equation of state

Aspen Plus Solution:

4.01. Start Aspen Plus V8.0. Select New | Blank and Recent | Blank Simulation. Click Create.

4.02. Define Components. Enter ethane and ethylene as Component ID’s, Aspen Plus will recognize these

components and will fill in the remaining component information.

4.03.

Define Property Methods. Click on Methods in the navigation pane. Select PENG-ROB as the Base

Method. The Peng-Robinson equation of state is typically used to model systems containing

hydrocarbons at high pressures. Populate the model parameters by clicking Methods | Parameters |

Binary Interaction | PRKBV-1. You can check the accuracy of these parameters by comparing predicted

values against data from the NIST Thermo Data Engine. You can also define new parameter values by

doing a data regression of experimental data.




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4.04. Construct the Flowsheet. Go to the simulation environment by clicking the Simulation button at the

bottom left of the screen. Place a RadFrac block onto the flowsheet and connect the feed, distillate, and

bottoms ports with material streams. The RadFrac block is in the Columns section of the Model Palette;it is a rigorous distillation model which can model multi-phase, non-ideal liquids, and rate-controlled

mixing. Rename streams as appropriate.




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4.05. Specify the Feed stream. Double click on the feed stream on the main flowsheet, or go to Streams |

FEED in the navigation pane. Enter the feed composition, flowrate, and state variables as shown below.




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4.06. Specify column operating conditions. Go to Blocks | C2SPLIT | Specifications | Setup. Enter 125 for

Number of stages and select Total for Condenser. The operating specifications must be determined

before you can run the simulation. It is common to specify Distillate to feed ratio and Reflux ratio.

Initial estimations will be used as first approximations, which can then be refined in order to meet

product specifications. We know what our feed flowrate is, and we know what our desired distillate

flowrate is, therefore we can specify the Distillate to feed ratio. In this case it is 0.684932 by mass. A

Reflux ratio of 4 will be entered as a first guess. This is shown below.

4.07. Next, go to the Streams tab and enter the stage number at which the feed stream will enter the column.

In this case, specify a stage number of 90.

(FAQ) Useful Option To Know: Convention for stage numbering

The numbering convention in Aspen Plus is that the condenser is stage 1, with stage numbers

increasing down the column.




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4.08. Next, go to the Pressure tab and enter a Stage 1/condenser pressure of 300 psig. For this problem, we

will assume that there are no pressure gradients throughout the column. If no pressure drop is entered

into Aspen Plus, the top stage pressure will be assumed to be the operating pressure of the entire

column. Note that in real life situations there will be a pressure drop throughout the column.




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4.09. Open the Control Panel and run the simulation (F5).

4.10. Create a custom stream table to view your results. Go to Blocks | C2SPLIT | Stream Results (Custom).

Right click on the Default tab and select Add New. Enter the sheet name for the new results table. Next,

right click on the new sheet tab that you just created and select Edit View. In the Stream Summary Edit

View Wizard that appears, select the streams, the properties, and the units you wish to display. This

process is shown below.




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4.11. Check results. In the new data sheet you created you can see that both ethylene and ethane streams

meet the product specifications.

4.12. View column profile. Go to Blocks | C2SPLIT | Profiles. Here you can view Temperature, Pressure, Heat,

and Material profiles over the column. Clicking the Compositions tab will display the compositions at

each stage in the column. You can choose to display either vapor or liquid compositions, and you can

choose to display mole or mass fractions. You can then graph these profiles by using the plot tool

located on the Home ribbon. Temperature and composition profile plots are shown below.




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5. Conclusions

The 125 stage column was able to exceed the specification of 99.96wt% ethylene at 5 million lb/day, as well as

the bottoms having less than 0.10wt% ethylene. It could then be concluded that this column is capable of

completing the desired separation. Aspen Plus allows engineers to model existing equipment and see if it ispossible to repurpose it or, otherwise, design new equipment that would meet very specific criteria.

6.

Copyright

Copyright © 2012 by Aspen Technology, Inc. (“AspenTech”). All rights reserved. This work may not be

reproduced or distributed in any form or by any means without the prior written consent of

AspenTech. ASPENTECH MAKES NO WARRANTY OR REPRESENTATION, EITHER EXPRESSED OR IMPLIED, WITH

RESPECT TO THIS WORK and assumes no liability for any errors or omissions. In no event will AspenTech be

liable to you for damages, including any loss of profits, lost savings, or other incidental or consequentialdamages arising out of the use of the information contained in, or the digital files supplied with or for use with,

this work. This work and its contents are provided for educational purposes only.

AspenTech®, aspenONE®, and the Aspen leaf logo, are trademarks of Aspen Technology, Inc.. Brands and

product names mentioned in this documentation are trademarks or service marks of thei r respective companies.

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